1. Field of the Invention
The present invention relates to an optical switch.
2. Description of the Related Art
With recent increases in the capacity of optical fiber communications, the bit rate of these communications has reached 160 Gb/s or higher and an optical switch that can adopt such speeds has been sought in research aiming at the next generation system. Among optical switches that are operated at ultra-high speed there exists an optical fiber switch employing an all-optical processing technique that controls light via another light and that utilizes a nonlinear optical effect in an optical fiber. Optical switches employing a highly nonlinear fiber, in particular, generally have characteristics of high switching efficiency and low loss.
One of the nonlinear optical effects applied to such an optical fiber switch is a phenomena referred to as four-wave mixing. Four-wave mixing is a phenomena in which third light (idler light) is generated when control light (λC) having the same wavelength as the zero-dispersion wavelength (λO) of the optical fiber and signal light of a wavelength different from the wavelength of the control light are input into an optical fiber. When that happens, gain of power is generated in the signal light like in the idler light. This is referred to as parametric gain.
FIG. 1 is a diagram showing an example of the typical configuration of the optical fiber switch utilizing four-wave mixing.
This configuration causes the polarization state of signal light with wavelength λS and the control light with wavelength λC to be parallel with each other using polarization controllers 10-1 and 10-2, respectively, multiplexing the lights using a directional coupler 11 such as an optical coupler, and inputting the light into an optical fiber 12 of a zero-dispersion wavelength λO.
In order to generate four-wave mixing efficiently, a phase matching condition of the signal light and the control light must be satisfied along through the highly nonlinear fiber. The phase matching condition is a condition in which the amount of phase mismatching Δβ of the signal light and the control light is 0, as shown in equation (1). Here, dD/dλ denotes the dispersion slope of the optical fiber [ps/nm2/km], c denotes the speed of light [m/s], and π denotes the ratio of the circumference of a circuit to its diameter.
                              Δ          ⁢                                          ⁢          β                =                              -                                          2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                c                                            λ                0                                              ⁢                                    ⅆ              D                                      ⅆ              λ                                ⁢                      (                                          λ                c                            -                              λ                0                                      )                    ⁢                                    (                                                λ                  c                                -                                  λ                  s                                            )                        2                                              Equation        ⁢                                  ⁢                  (          1          )                    
As is clear from equation (1), if the wavelength of the control light λC and the zero-dispersion wavelength λO of the optical fiber are the same, the amount of phase mismatch Δβ is 0. Accordingly, if the wavelength of the control light λC and the zero-dispersion wavelength λO of the optical fiber are the same at any place over the length of the optical fiber in the longitudinal direction, the highest efficiency four-wave mixing can be generated.
The definitions of generation efficiency η and the bandwidth of four-wave mixing are given next. The four-wave mixing efficiency η is defined as PI/PS, which is the ratio of the idler light optical power (PI) at the optical fiber output end to the signal light optical power (PS) at the input end of the optical fiber. The efficiency is, as shown in equation (2), a value proportional to the squared product of a nonlinear coefficient of an optical fiber γ[W−1 km−1], a length of the optical fiber L[km], and the peak power of the control light (PC).η∝(γPCL)2  Equation (2)
Here, the bandwidth of four-wave mixing is defined as the bandwidth of the signal light where the maximum generation efficiency is reduced by half (reduced by 3 dB) by measuring the generation efficiency of four-wave mixing when the wavelength of the signal light (λS) is shorter than that of the control light (λC) (λS<λC), and when the wavelength of the signal light (λS) is longer than that of the control light (λC) (λS>λC).
When four-wave mixing is utilized in an optical switch, it is desirable to have a high generation efficiency and to operate over a broad bandwidth. If the phase mismatch Δβ is 0 at any place, the phase matching condition is satisfied in any wavelength of the signal light, and the operation bandwidth increases to infinity. In order to satisfy the phase matching condition, the dispersion slope of the optical fiber has to be 0 at any place, or the wavelength of the control light (λC) and the zero-dispersion wavelength (λO) of the optical fiber have to match completely. It is clear from equation (2) that the generation of high-efficiency four-wave mixing requires an increase in the nonlinear coefficient, enhancement of the peak power of the control light, and an increase in the length of the optical fiber. These characteristics are the same for the parametric gain to the signal light.
However, it is difficult to obtain a dispersion slope of exactly 0 in actual optical fibers, and the zero-dispersion wavelength fluctuates along the length of the optical fibers in the longitudinal direction. For example, the measurement result of the zero-dispersion wavelength in the longitudinal direction of an optical fiber is reported in Non-patent Document 1, and that of the highly nonlinear fiber is reported in Non-patent Document 2.
FIG. 2 is a diagram showing the distribution of the zero-dispersion wavelength measured along the length of the highly nonlinear fiber in the longitudinal direction, reported in Non-patent Document 2.
Fiber-A and Fiber-B in FIG. 2 are samples of highly nonlinear fibers having different nonlinear coefficients. It is reported that both fibers have zero-dispersion wavelengths that fluctuate by approximately 2 nm in 2 km of an optical fiber. Although explained later, the fluctuation state of the zero-dispersion wavelength is first described here. “Sample” in FIG. 2 indicates a 100 m sample cut out from the whole. From FIG. 2, many samples which have a zero-dispersion wavelength that monotonically increases (or monotonically decreases) in the longitudinal direction can be cut out by limiting their length to several hundred meters.
As explained above, in conventional optical fibers the phase matching condition cannot be satisfied completely because the zero-dispersion wavelength fluctuates in the longitudinal direction, and this is one of the factors that limit the generation efficiency and bandwidth of four-wave mixing.
Conventional optical devices employing four-wave mixing are described in Patent Documents 1 and 2, and these documents disclose a technology for reducing the polarization dependency of optical devices which utilize four-wave mixing.
[Non-patent Document 1]
L. F. Mollenauer, et al., Tech. Digest, OFC' 97 pp. 255-256
[Non-patent Document 2]
Hirano et al., OFC 2005 Post deadline session 4
[Patent Document 1]
Japanese Patent Application Publication No. 05-289124
[Patent Document 2]
Japanese Patent Application Publication No. 11-238941
In order to solve the above issue, the conventional method reduces the fluctuation range of the zero-dispersion wavelength by shortening the length of the highly linear fiber to obtain a wider operation band bandwidth; however, this causes a problem in which the generation efficiency is lowered. On the other hand, if the length L of the optical fiber is extended in order to enhance the efficiency, a problem occurs in which the bandwidth becomes narrow and the bandwidth and the efficiency therefore enter a trade-off relation, as shown in FIG. 7.
FIG. 3 is a diagram showing the concept of the trade-off relation between the bandwidth and the efficiency.
As shown in FIG. 3, when a comparison is made under the same power conditions, if the length L of an optical fiber is short, as indicated by the dotted line, the efficiency η is low but has a wide bandwidth. In addition, if the length L of the optical fiber is long, as shown by the solid line, the bandwidth becomes narrow but has a high efficiency η.